The present invention relates in general to the heat treatment of metals and, more particularly, to the heat treatment of superalloy workpieces in which there is a conflict between the desired properties of the center versus those of the periphery.
In the gas turbine engine industry, to which the present invention has particular application, the achievement of absolute maximum engine performance is often limited by the properties of the materials from which the engines are fabricated. The art and science of alloying and treatment of metals to maximize desired properties has developed to a high level of sophistication. These advances in technology can be frustrated, however, when the properties which must be maximized are contradictory, that is, when the maximizing of one property necessarily precludes maximizing another important property. Where the contradictory properties must exist in the same location in the workpiece, a compromise and some sacrifice of one or both of the properties are almost inevitable.
There is one type of situation where the compromise may be avoided. This is when one of the contradictory properties must exist in a first portion of the workpiece and the other contradictory property must exist in another portion of the workpiece. Theoretically, one need only process the workpiece to maximize the properties appropriate to each portion of the workpiece. There are, however, practical problems with achieving this result.
One situation where the contradictory properties exist on separate portions of the workpiece concerns gas turbine engine discs. In operation, these discs are exposed to extremes and cycles of high temperatures and force, as they rotate at speeds in the order of 10,000 rpm. Air foils are dove-tailed into the periphery of the disc and extend radially from the disc. As the turbine rotates at high speed, the dovetail joint between the periphery of the disc and each blade is exposed to extreme stress. Because the periphery is the part of the disc which is exposed to the highest temperatures, high temperature creep resistance becomes a critical property. Its upper limit, in terms of temperature and force, places a limit on the efficiency and capacity of the turbine of which the disk is a part. Generally, in the alloys used in this-application, a coarse grain structure possesses higher creep resistance than a fine grain structure.
The problem of contradictory properties exists in the disc because, whereas the periphery should have relatively coarse grain, the central portion should have relatively fine grain. To maximize the efficiency and capacity of a gas turbine, it is desirable to maximize the tensile strength and low cycle fatigue resistance of the central portion of the discs. This is achieved by relatively fine grain sizes.
The problem of contradictory properties in gas turbine discs is often further complicated by the fact that, in high performance turbines, it is often the case that reliability must approach 100%, because of the consequences of disc failure. Destructive testing becomes impractical, yet nondestructive testing is often not sufficiently indicative of flaws. Thus, practical manufacture of the turbine discs must often rely on the consistent predictability of the results of the processing. This means that the result of the processing must not only be inherently predictable, but also, the process must be capable of being efficiently monitored to be sure that it is being carried out in accordance with the specifications.
With regard to the turbine disc problem, several approaches could be considered. First, the disc could be formed in two pieces; an inner ring and an outer ring. Each could be processed to have its own properties. Then the rings could be welded or diffusion bonded together. Both welding and diffusion bonding, however, can have serious predictability problems and can cause undesirable changes in the properties of the workpieces.
Second, it would be possible to heat treat the whole disc to a coarse state and then forge the central portion to reduce the grain and cause work hardening. This approach can have predictability problems, would be very complicated to carry out effectively, and would provide poor control of the nature and location of the interface between the two portions. Furthermore, the tooling necessary to carry out the process would be very expensive to develop and would be limited to a specific workpiece and a specific set of properties, i.e., it would be inflexible.
Third, it would be possible to treat the entire disc to form fine grain and then to heat the periphery to coarsen the grain by induction heating. It was found that induction heating can be a very imprecise way of heating a specific portion of the disc and could not always be sufficiently controlled to provide reproducible results, with respect to both grain size and interface nature and location.
These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a dual-property workpiece system which can be implemented with a new combination of relatively simple, time-tested equipment and operations.
Another object of this invention is the provision of a dual-property workpiece system which provides precisely predictable results.
A further object of the present invention is the provision of a dual-property workpiece system in which the variables can be precisely controlled, monitored and recorded.
It is another object of the instant invention to provide a dual-property workpiece system which can be easily and inexpensively tailored to create a wide range of results in terms to geometry and properties.
A still further object of the invention is the provision of a dual-property workpiece system which is fully compatible with other operations on the workpiece.
It is a further object of the invention to provide a dual-property workpiece system which can create workpieces having geometric property distributions and gradations not practically possible with any other system.
With the foregoing and other objects in view, which will appear as the description proceeds, the invention resides in the combination and arrangement of steps and the details of the structure hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.